Much of modern technology makes use of thin solid films on the surfaces of solid substrates. A number of methods have been used to deposit such thin films including thermal evaporation, DC sputtering, RF sputtering, ion beam deposition, chemical vapor deposition, plating, molecular beam deposition and deposition from the liquid phase.
The structure of thin films can be amorphous (that is, the atoms of the film are not arranged in any crystalline order), randomly polycrystalline (that is, the film is composed of many small regions, in each of which the atoms are arranged in a regular crystalline order, but the small regions have no mutual alignment of their crystallographic axes), textured-polycrystalline (that is, the film is composed of many small regions, in each of which the atoms are arranged in a regular crystalline order, and one or more of the crystalline axes of the majority of said regions are parallel), or epitaxial (that is, the film is predominantly of a single crystallographic orientation). An epitaxial or nearly single crystal film is a special case of a preferred orientation film in which corresponding crystallographic axes of all the small regions are essentially oriented in the same directions. A thin film can be the same material (that is, the same element or compound) as the substrate (producing a "homogeneous" structure), or it can differ in chemical composition from the substrate (producing a heterogeneous structure). If the film is epitaxial, the former is called "homoepitaxy" and the latter "heteroepitaxy".
By "crystallization" is meant the process of arranging the atoms of a substance in a crystalline order. For convenience, the term should also be understood to encompass "recrystallization" as well, when referring to a substance which already has some degree of crystalline order, in which case, the atoms are arranged in a higher crystalline order by "recrystallization".
In the process of fabricating thin films on a substrate, the films may be subjected to forces which induce stress in the film. For example, under certain conditions, when heat is applied to materials (as usually occurs in the process of fabricating thin films) thermal stress can be induced in the film.
The stress "S" induced in a thin film (Material 1) supported by a substrate (Material 2) is essentially "two dimensional" and may be approximated by Equation 1 below: EQU Equation 1 S.varies.(.alpha..sub.1 .DELTA.T.sub.1 -.alpha..sub.2 .DELTA.T.sub.2)
wherein
.DELTA.T.sub.1 =T.sub.1 -T.sub.0 PA1 .DELTA.T.sub.2 =T.sub.2-T.sub.0 PA1 T.sub.0 =a reference temperature, normally near room temperature, at which the device is intended to operate. PA1 T.sub.1 and T.sub.2 =the highest temperature to which the materials 1 and 2 are subjected to but not greater than the temperature at which the respective materials become almost molten or plastic. PA1 .alpha..sub.1 and .alpha..sub.2 =the thermal coefficients of expansion of materials 1 and 2, respectively, expressed in (.degree.C.).sup.-1.
In connection with Equation 1, the following should be noted:
(1) In cases wherein the materials are subjected to a temperature which cause the materials to become plastic, this Equation 1 will have to be modified slightly but still remains a fair approximation.
(2) Often .alpha..sub.1, and .alpha..sub.2 slowly vary with temperature, but for purposes of this approximation .alpha..sub.1 and .alpha..sub.2 are assumed to be constants.
(3) Material 2 is usually much thicker than Material 1 and may be a composite of two or more materials.
(4) The reason the stress may be considered as two dimensional is that the film is much thinner than the substrate.
In a typical heterogenous structure, such as an SOS structure involving the formation of a film of silicon on sapphire (Al.sub.2 O.sub.3), by chemical vapor deposition (CVD), both the substrate and film are subjected to the same temperature cycle, in which case, Equation 1 reduces to: EQU Equation 1 S.varies.(.alpha..sub.1 -.alpha..sub.2) .DELTA.T where: EQU .DELTA.T.sub.1 =.DELTA.T.sub.2 =.DELTA.T
In a typical SOS CVD process .DELTA.T is about 1000.degree. C. and .alpha..sub.2 =2 .alpha..sub.1 where .alpha..sub.1 is the thermal expansion coefficient for Si and .alpha..sub.2, the thermal expansion coefficient of sapphire. In this example, the stress in the Si film, S, will be a negative number indicating that the film will be subjected to a compressive stress.
Those skilled in the art teach that compressive thermal stress in Si on sapphire devices is an undesirable condition that should be avoided or at least minimized inferring that such stress reduces the electron mobility of the Si film. More specifically, Hyneak, in an article entitled "Elastoresistance of n-Type Silicon on Sapphire" Journal of Applied Physics V45 No. 6 June 74, studied the influence of stress on the electrical properties of SOS. He concluded, in general terms, that "the electron Hall mobility of the silicon on sapphire is always measured smaller than its corresponding bulk value by a factor of 2. Thus, by reducing the stress in SOS, a significant improvement in electrical properties-could be obtained".
Sai-Halasz et al. in a paper entitled "Stress Relieved Regrowth of Silicon on Sapphire by Laser Annealing" Appl. Phys. Lett. 36(6) March 15, 1980, have considered this stress phenomena in SOS and proposed a solution which might minimize the adverse effect of the compressive strain induced in the Si film during CVD epitaxial growth of Si on sapphire. Their proposed solution is to subject the film and substrate to different temperatures by laser annealing the Si film. In this case, Equation 2 above is not applicable since .DELTA.T.sub.1 no longer is equal to .DELTA.T.sub.2. Instead, they make .DELTA.T.sub.1 much greater than .DELTA.T.sub.2 to offset the fact that .alpha..sub.1 is much less than .alpha..sub.2 : thereby balancing out the effect of the different thermal expansion coefficients.
In homogeneous structures, where the film and substrate material are identical, thermally induced stress is usually not a factor provided both the film and substrate are subjected to the same temperature profile. This is for the reason that in the homogeneous case, .alpha..sub.1 =.alpha..sub.2 and if .DELTA.T.sub.1 also equals .DELTA.T.sub.2, according to Equation 1, S will equal zero.
The above examples relate to the background art status of planar or substantially "two dimensional" stress wherein the emphasis and objectives have been directed toward minimizing or reducing stress in the structure. Additionally, the background art comprises research into the effects of uniaxial (one dimensional) stress and hydrostatic (three dimensional) stress on the electrical properties of crystalline boules (bulk material).
For example, C. W. Smith, Phys. Rev. V94, 42, 1954, reports that uniaxial tension changes resistivity in bulk Si and Ge and quantifies and results of experiments on these materials. The prior art experiments show that the change in resistivity in bulk material is related to the type of doping i.e., n (electron) or p (holes) and the direction of the stress, i.e., compressive or tensile. Tensile stress decreases the mobility of holes and compressive stress decreases the mobility of electrons.